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Re: Confusion and Questions about blocks read

From: Jim Nasby <jim(at)nasby(dot)net>
To: Markus Schaber <schabi(at)logix-tt(dot)com>
Cc: "Pgsql-Performance ((E-mail))" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Confusion and Questions about blocks read
Date: 2006-09-27 03:08:35
Message-ID: FD014885-5492-4F44-8C2E-F77C85B4FDCF@nasby.net (view raw or flat)
Thread:
Lists: pgsql-performance
On Sep 23, 2006, at 8:19 AM, Markus Schaber wrote:
> Btw, would it be feasible to enhance normal index scans by looking at
> all rows in the current table block whether they meet the query
> criteria, fetch them all, and blacklist the block for further  
> revisiting
> during the same index scan?
>
> I think that, for non-sorted cases, this could improve index scans a
> little, but I don't know whether it's worth the effort, given that
> bitmap indidex scans exist.

The trade-off is you'd burn a lot more CPU on those pages. What might  
be interesting would be collapsing bitmap scan data down to a page  
level when certain conditions were met, such as if you're getting a  
significant number of hits for a given page. There's probably other  
criteria that could be used as well. One issue would be considering  
the effects of other bitmap index operations; if you're ANDing a  
bunch of scans together, you're likely to have far fewer tuples per  
page coming out the backside, which means you probably wouldn't want  
to burn the extra CPU to do full page scans.

BTW, I remember discussion at some point about ordering the results  
of a bitmap scan by page/tuple ID, which would essentially do what  
you're talking about. I don't know if it actually happened or not,  
though.

If this is something that interests you, I recommend taking a look at  
the code; it's generally not too hard to read through thanks to all  
the comments.
--
Jim Nasby                                    jimn(at)enterprisedb(dot)com
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)




--
Jim Nasby                                    jimn(at)enterprisedb(dot)com
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)



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